Cockney rhyming slang
(see main article at Cockney rhyming slang)
While not a priority language item for most EFL students - or teachers - those students planning to spend time in or around London will inevitably come up against these binomial expressions at some stage. Useful for linking exercises and, of course, rhyming.
Rhyming slang works by replacing the word to be obscured, for instance "look" (as in "Let's have a look"), with the first word of a phrase that rhymes with that word. Thus, "look" would be replaced by "butcher's," because "look" rhymes with the phrase "butcher's hook": "Let's have a butcher's".
For good or for evil the internet has generated a whole new slang language which can make some sites and forums almost unintelligible to those not in the know. There is even an internet slang translator for those not in the know, and which is run by noslang.com.
The extensive use of lolspeak, emoticons and initials, many of which are totally unintelligible to the layperson, falls into this category, and some students, especially those receiving emails from abroad, might need an intensive course in some of the more frequent ones.